Animal Diversity 4: Cambrian Explosion

Describe the contents and interpretation of the Burgess Shale

Interpretation

• Variety of invertebrate fossils

• Most related to modern groups

• ‘Walcotts’s shoehorn’

• Largely neglected after

Biota:

• Porifera, Ctenophora, Cnideria

• Annelida and Mollusca

• Priapulida and Onychophora

• Arthropoda such as ANOMALOCARIS

• Deuterostomia such as Metaspriggina

Define a phylum

• A phylum is a unique body plan

• There are around 30 modern animal phyla

• Most are bilaterians

• Arthropoda: Bilateria with jointed appendages

• Chordata: Bilateria with notochord and dorsal nerve chord

 

Define a crown and stem group

• The crown group is a living member of a clade, its common ancestor, and all descendants

• The stem group are extinct organisms outside that group, on the lineage to that most recent common ancestor

• E.g. Crown group Aves (birds), total group Aves (dinosaurs), Stem group Aves (both), Crown group Archosauria (crocodilia)

• Many of the Cambrian Burgess Shale fossil taxa are interpreted as stem-groups of modern phyla

What happened to the rate of evolution during the Cambrian

• An analysis of rates of arthropod evolution in the Cambrian and post-Cambrian

• A massive phylogeny of arthropods using lots of molecular and morphological data

• Evolution was about 5x faster than normal during the Cambrian

Why was the Cambrian explosion significant at the phylum level

• Most animal phyla first appear for the first time in a sudden burst of diversity: the Cambrian Explosion, very few phyla have a later origin

• The diverse and disparate body plans of animals were therefore established in a very small period of time, at the beginning of the Cambrian (or earlier)

What are the possible biological causes of the Cambrian explosion

Biological

• More complex ecological interactions: Predator-prey “arms race”

• Cambrian ecosystems were more complex than before: predators and prey with complex visual and movement systems

• Developmental “Explosion” of novelty: Newer body plans were easier to develop earlier on without constraints

• Animals all share a common fundamental mechanism by which they develop and grow: the pattern in which Hox genes are expressed in the developing embryo

• Hox genes specify, for example, the position of head and appendages

• Earlier on, when there is no competition and no constraints, lots of different body plans might rapidly originate using same developmental mechanisms

What are the possible geological causes of the Cambrian explosion

Geological

• The arrangement of continents changes at beginning of Cambrian

• Breakup of Rodina supercontinent and lots of exposed shelf: habitat for animals

• Large amount of exposed continental rock leads to much larger amount of weathering and carbonate input into the oceans and increasing ocean alkalinity

Describe Burgess Shale type preservation

Characteristics

• Deep seabed rapidly buried by fine sediment

• Starved of oxygen, potentially slowing bacterial decay

• Metamorphosed

• Chemical composition of clays

Causes

• Big change in Ediacaran and Cambrian environments

• Soft tissues are only preserved in certain extra fossiliferous layers

• Low sulphur and high alkalinity thought to be behind “opening of taphonomic window”

• We don’t see it after the Cambrian